docs: def www_authenticate(self):
          """The `WWW-Authenticate` header in a parsed form."""
          def on_update(www_auth):
              if not www_auth and 'www-authenticate' in self.headers:
                  del self.headers['www-authenticate']
              elif www_auth:
                  self.headers['WWW-Authenticate'] = www_auth.to_header()
          header = self.headers.get('www-authenticate')
          return parse_www_authenticate_header(header, on_update)